1. Field of the Invention
The present invention relates to data repeaters, and particularly to a data repeater for repeating plural data types, and a multiplex communication system using the same.
2. Related Art
Recently, data communication has become more sophisticated particularly in the area of computer technology. In the automotive field, for example, an amount of data communicated within a control portion for controlling electrical equipments to be mounted on the vehicle has rapidly increased. Therefore, a multiplex communication system must be applied in order to reduce the number of wire harnesses for transmitting the data. The communication system performs data communication via a multiplex communication lime between communication nodes; by connecting the communication nodes such as a controlling Electronic Control Unit (ECU) for sending or receiving data to a common multiplex communication line. The data is constructed by containing information such as type of the data in addition to primary data. In the case of a vehicle in which the number of types of control is large, a plurality of communication groups, each of which is made up of a multiplex communication line and a node to be connected to this communication line, is provided based on a difference in a communication speed to be required in order to effectively perform the data communication. The communication between nodes whose communication groups are different from each other is performed via a data repeater.
FIG. 37 shows such a multiplex communication system having a plurality of communication groups. A data repeater 8 of the multiplex communication system has receiving buffers 811, 812, 813 and sending buffers 821, 822, 823 individually provided to each multiplex communication lines 61, 62, 63 of communication groups 51, 52, 53, and a data controller 83. In the case where data transferred to one of the communication groups 51-53 is different from the communication groups 51-53 to which a communication node 71, 72, 73 having sent the data belongs, the multiplex communication system operates as follows. That is, data sent from the communication node 71 connected to the first multiplex communication line 61 to the communication node 72 connected to the second multiplex communication line 62 is rewritten to the receiving buffer 811 for the first multiplex communication line in the data repeater 8. The data controller 83 obtains information that the data should be sent from the receiving buffer 811 to the second multiplex communication line 62. The data in the receiving buffer 811 is transferred to the sending buffer 822 for the second multiplex communication line. The data transferred to the sending buffer 822 is sent to the second multiplex communication line 62 at a predetermined timing (First prior art).
Furthermore, as shown in FIG. 38, receiving buffers 811a, 811b for the first multiplex communication line, receiving buffers 812a, 812b for the second multiplex communication line, receiving buffers 813a, 813b for the third multiplex communication line, sending buffers 821a, 821b for the first multiplex communication line, sending buffers 822a, 822b for the second multiplex communication line, and sending buffers 823a, 823b for the third multiplex communication line are respectively connected in serial in the data repeater so that these buffers are used as an FIFO (First In First Out) type. The data repeater 8A, can hold a large amount of data by waiting to send data even when the data received early is stacked in the last sending buffers 821b-823b (Second prior art).
However, even if the data remains at the sending buffer, other data to be transferred is sent to the data repeater regardless of such a situation. Therefore, in the case the receiving buffer receives the data when the receiving buffer has data already written therein, the data in the receiving buffer is overwritten by the newly received data so that the previously existing data is lost. In this case, it would be not problem when the previously written data is overwritten by the same type of data so as to be updated. However, it is problem when the data is overwritten by another type of data because certain type of data is completely lost.
According to the second prior art, there are some spare buffers compared to the first prior art because the number of the receiving buffers and the sending buffers is larger than that of the first prior art. However, when the sending to the multiplex communication line from the sending buffer is delayed because the multiplex communication line to be sent is busy, the data is ultimately lost, and therefore a reliability of data communication decreases. Here, it might be thought to considerably increase the number of receiving buffers or the sending buffers to be arranged in serial in the second prior art. However, the construction becomes complicated and it leads to an increase of overall system cost.